Continuous tube making process lines with means to provide an external galvanized coating are well known. For example, reference may be had to Krengel U.S. Pat. Nos. 3,230,615; 3,231,708; 3,256,592 and 3,561,096 and Rossi U.S. Pat. No. 3,908,593 for continuous tube forming and galvanizing disclosures.
The continuously formed galvanized tube is then cut into predetermined lengths for a number of end uses. For example, these lengths may be installed in an electrical wiring system in which the wires run through the insides of the assembled lengths of conduit. Prior to, during and after installation, the internal diameter of the conduit may be exposed to atmospheric or soil conditions, with potential oxidation or corrosion of uncovered tube surfaces being possible. To avoid or minimize such oxidation or corrosion, the internal diameter of the tube is preferably covered with a protective coating to increase its operational life.
Two methods of obtaining a tube having a painted internal diameter are disclosed in Ostrowski U.S. Pat. No. 3,768,145. In the first method, a lance is supported within the formed tube, with the nozzle end thereof being positioned downstream of the weld station and being held in a centered position by spring spacers extending between the lance and the internal tube wall. The nozzle end of the lance has circumferentially spaced spray orifices to discharge paint throughout a 360.degree. extent to coat the entire internal surface of the formed tube.
In the second disclosed method, the flat upper surface of the steel strip, which will become the internal diameter of the tube after forming, is painted in its flat form prior to the forming and welding steps. After welding, the nozzle end of the lance supported by the spring members has an orifice to direct paint to the weld area to apply a "touch up" coat to the seam.
In both modes of operation disclosed in the Ostrowski patent, paint is being applied to the internal diameter of the formed tube. In addition, in the second mode, in which the strip is painted in its flat form prior to welding, the spring loaded centering device is rubbing against the moving tube to possibly abrade or mar the painted surface.
The use of precoated or prepainted steel strip is increasing. However, when the precoated strip is welded, the heat of the welding process and the weld bead formed effectively remove the internal coating at and immediately adjacent to the seam. A lance as shown in the Ostrowski patent could be used to paint the internal weld bead area. However, the spring support and centering mechanism in Ostrowski might abrade or scratch the precoated surface and any paint applied may not be compatible with or as good as the preapplied coating. The spring support and centering mechanism may also be subject to breakage or warpage caused by the heat generated from welding and friction and is also subject to relatively frequent maintenance or replacement.